A boiler in which coal is burned emits flue gas containing a large amount of sulfur oxides such as SO.sub.2. Therefore, it is the common practice to pass the flue gas through a desulfurization tower before the gas is discharged to the atmosphere to remove the sulfur oxides, for preventing public nuisance such as acid rain.
A conventional desulfurization tower which acts as a gas absorption tower for removing sulfur oxides is constructed as shown in FIG. 6, where a primary desulfurization tower and a secondary desulfurization tower are indicated by numerals 10 and 20, respectively. Waste gas is first supplied to the primary desulfurization tower 10, where most of the sulfur oxides is removed. The flue(waste) gas is introduced into the secondary desulfurization tower 20, where the remaining sulfur oxides is removed up to a given value.
The primary desulfurization tower 10 is rectangular in cross section and extends vertically. The tower 10 has a flue gas inlet port 11 at its top position and a flue gas exit port 13 at its lowest position. The exit port 13 acts also as an entrance port to the secondary desulfurization tower 20. The exit port 13 opens into the lower portion of the secondary tower 20 and is connected to it. The primary tower 10 is rigidly fixed to foundations by a support leg 19. A number of nozzles 12 for spraying absorbent are mounted inside the primary tower 10 in such a way that they are vertically regularly spaced from each other. The nozzles 12 are disposed opposite to the flow of the waste gas. A vessel 21 for holding(receiving) the absorbent is mounted at the bottom of the secondary tower 20. A pipe 16 extends from the vessel 21 to the nozzles 12 via a pipe 14 and a pump 15. A make-up liquid supply pipe 62 is disposed in the pipe 14 on the suction side of the pump 15. A discharge pipe 17 discharges a part of used absorbent (absorbing liquid) to waste water treatment equipment (not shown).
The secondary desulfurization tower 20 is circular in cross section and extends vertically. The lower portion of the tower 20 is rigidly fixed to foundations by a support leg 35. The aforementioned vessel 21 for receiving the absorbing liquid is mounted at the bottom of the secondary tower 20 below the flue gas exit port 13 of the primary tower 10. A vapor-liquid separation chamber 22 is formed above the vessel 21 and faces the opening of the flue gas exit port 13 of the primary tower 10. Vessels 23 are mounted above the chamber 22 to receive absorbent dropping from a secondary desulfurization section located at a higher position. A plurality of chimneys 24 which open at their lower ends into the chamber 22 extend through each vessel 23. Umbrellas 24a are mounted above each chimney 24 so as to form gas discharge ports. Each umbrella 24a is equipped with smaller chimneys 24b which are similar in structure to the chimneys 24. A packed bed 25 composed of resin particle materials which brings gas into contact with the absorbent is mounted above the vessels 23. A multiplicity of nozzles 31 are mounted above the packed bed 25 to spray the absorbent. An eliminator 26 for removing mist such as liquid droplets entrained by gas is mounted above the nozzles 31. A discharge port 27 for discharging desulfurized gas is formed at the top of the tower 20. A pipe 30 extends from the vessels 23 to the nozzles 31 via a pipe 28 and a pump 29. A make-up liquid supply pipe 32 is connected to the pipe 28 on the suction side of the pump 29.
The operation of the desulfurization tower constructed as described thus far is now described. Flue gas from a boiler is at a temperature of 140.degree. C. to 150.degree. C. and contains 1000 ppm of SO.sub.2, for example. The flue gas is supplied into the primary desulfurization tower 10 from the waste gas supply port 11 of the tower 10. The gas makes contact with a counterflow of an absorbing liquid sprayed from the nozzles 12 while traveling downward. Thus, most of the sulfur oxides such as SO.sub.2 contained in the waste gas is removed. At this time, dust such as fly ashes contained in the flue gas is also removed, and the flue gas is cooled. When the flue gas treated in this way leaves the flue gas exit port 13 of the primary desulfurization tower 10, the temperature is approximately 60.degree. C., and the concentration of SO.sub.2 is about 150 ppm, for example. After the flue gas undergoes the primary desulfurization, it is introduced into the vapor-liquid separation chamber 22 in the secondary desulfurization tower 20 from the flue gas exit port 13. In this chamber, liquid is separated from gas. The liquid is held in the lowermost vessel 21. The gas, or waste gas, flows upward through the tower 20, passes through the chimneys 24, the smaller chimneys 24b, and are uniformly distributed over the cross section of the tower 20. Then, the gas is admitted into the packed bed 25 located at a higher position. Inside the packed bed 25, the absorbing liquid sprayed from the nozzles 31 makes a uniform contact with the flue gas. As a result, SO.sub.2 of the gas is removed up to about 40 ppm, for example. The absorbing liquid which was passed through the bed 25 enters the vessels 23 and stays in them. The liquid is sent back to the nozzles 31 via the pipes 28 and 30 by the pump 29, so that the liquid is reused. The absorbing liquid overflows from the vessels 23 and falls into the lower vessel 21 of the primary desulfurization section. After SO.sub.2 of the flue gas is removed up to a given value by the filler 25, the gas is passed through the eliminator 26 to remove mist. Subsequently, the gas is discharged from the port 27.
The absorbing liquid which is used to remove SO.sub.2 (sulfur dioxide) can be a mixture of MgSO.sub.3 (magnesium sulfite), Mg(HSO.sub.3).sub.2 (magnesium hydrogensulfite), MgSO.sub.4 (magnesium sulfate), and H.sub.2 O (water). SO.sub.2 is absorbed chiefly by MgSO.sub.3. Mg(OH).sub.2 (magnesium hydroxide) is supplied to the mixture water solution. As an example, a mixture water solution having a concentration of 5% and a pH of 5.2 is employed in the primary desulfurization tower 10. A mixture water solution having a concentration of 1% and a pH of 5.9 to 6.2 is used in the secondary desulfurization tower 20.
In order to secure certain desulfurization performance, the flow rate of the gas treated in the desulfurization sections, especially in the primary desulfurization section, must be kept at a certain value. In the above-described conventional desulfurization equipment, the primary and secondary desulfurization towers are mounted separately and so the whole equipment is large. Especially, where a large flow rate of waste gas, e.g., 350,000 Nm.sup.3 /hr., is treated, the cross section of the primary desulfurization tower 10 measures 2 m by 4 m. The inside diameter of the secondary desulfurization tower 20 is as long as 8 m. Thus, both primary and secondary desulfurization towers which are large in this way are needed. Where a large amount of flue gas is treated, large and complex equipment is necessary. Also, large space is required to install it. This frequently imposes limitations on the installation. The secondary desulfurization tower 20 necessitates the absorbing liquid-holding vessel 23 equipped with the chimneys 24. This complicates the structure and increases the cost.